1. Field of the Invention
The present invention relates to a process for the preparation of alkyl nitrites from nitrogen dioxide and alcohols in a counterflow-operated column.
Alkyl nitrites (alkyl esters of nitrous acid) have a wide variety of uses, for example, as additives for engine oils, as stabilizers for unsaturated organic compounds, as spasmolytics, as reagents for oximations, nitrosations and diazotizations and as auxiliary materials for chemical syntheses.
2. Description of the Related Art
The most common way, which can be easily carried out in the laboratory, of preparing alkyl nitrites is the reaction of sodium nitrite with strong acids, for example with sulphuric acid, in the presence of the appropriate alcohol: EQU 2 NaNO.sub.2 +2 ROH+H.sub.2 SO.sub.4 .fwdarw.2 RONO+Na.sub.2 SO.sub.4 +2H.sub.2 O (1)
The process according to equation (1) is based on the high esterification rate of nitrous acid with the alcohol, it being possible to choose the reaction conditions such that the resulting alkyl nitrite can be removed from the reaction equilibrium as a gas, if appropriate by distilling off. The disadvantages of this process, however, lie in the use of expensive starting materials, such as, for example, sodium nitrite, and in the inevitable occurrence of large quantities of non-usable inorganic salts, in particular sodium sulphate. Furthermore, the alcohol used cannot in general be successfully brought to complete reaction, so that either corresponding losses have to be taken into account or a complicated treatment of the resulting waste water must be carried out.
Another process for preparing alkyl nitrites is based on the use of nitrogen oxides. In this case, preferably nitrogen monoxide, oxygen and the appropriate alcohol are caused to react with another, the following reactions taking place: EQU 2 NO+O.sub.2 .fwdarw.2 NO.sub.3 ( 2) EQU NO+NO.sub.2 .revreaction.N.sub.2 O.sub.3 ( 3) EQU ROH+N.sub.2 O.sub.3 .fwdarw.RONO+HNO.sub.2 ( 4) EQU ROH+HNO.sub.2 .fwdarw.RONO+H.sub.2 O (5) EQU N.sub.2 O.sub.3 +H.sub.2 O.fwdarw.2 HNO.sub.2 ( 6)
Here, it is attempted to allow the preparation of the sought-after alkyl nitrites to take place as far as possible only according to the reaction equations (2) to (5), water being obtained as the only waste substance. However, the reaction given in the reaction equation (6) is generally unavoidable, since the dinitrogen trioxide formed according to reaction equation (3) cannot only react with alcohol in the sense of equation (4) but can also react with the water produced according to equation (5). In the presence of adequate, in particular excess, quantities of the alcohol, however, the nitrous acid thus produced in the sense of equation (5) can be trapped to form the desired alkyl nitrite and water, and is thus not lost as a waste substance. A process taking place according to the described reactions is described, for example, in U.S. Pat. No. 2,831,882, where, for the preparation of isopropyl nitrite, nitrogen, nitrogen monoxide, nitrogen dioxide and isopropanol are introduced into a reaction vessel. German Patent Specification 11 56 775 describes the preparation of methyl nitrite by mixing nitrogen monoxide and oxygen in the ratio of 4,4:1 and introducing the gas mixture thus obtained into liquid methanol. U.S. Pat. No. 4,353,843 describes the reaction of a mixture of nitrogen monoxide and nitrogen dioxide, the proportion of the nitrogen monoxide being greater than that of the nitrogen dioxide, with gaseous methanol. EP 310 191, finally, describes the formation of lower alkyl nitrites by reaction of nitrogen monoxide, oxygen and the corresponding alcohols in a specific apparatus containing a reaction zone and a rectification zone, the water occurring in the course of the alkyl nitrite preparation being continuously removed by scrubbing out of the rising stream of product gas. The disadvantage of this process is the use of the nitrogen monoxide, which is technically poorly available in pure form. A further disadvantage is that, in order to suppress the formation, undesired in the process of EP 310 191, of nitric acid which results from the action of secondary reactions according to equations (7) to (9), excesses of nitrogen monoxide generally have to be used: EQU 2 NO.sub.2 .revreaction.N.sub.2 O.sub.4 ( 7) EQU ROH+N.sub.2 O.sub.4 .fwdarw.RONO+HNO.sub.3 ( 8) EQU N.sub.2 O.sub.4 +H.sub.2 O.fwdarw.HNO.sub.2 +HNO.sub.3 ( 9)
U.S. Pat. No. 2,739,166 describes a process according to which alkyl nitrites are obtained during the passing of gaseous nitrogen dioxide through the appropriate alcohols. Here, also according to the reaction equations (7) to (8), the desired alkyl nitrite is obtained mixed with nitric acid and unreacted excess alcohol. Basically, a procedure of this kind provides an advantage, since it permits the use of nitrogen dioxide, which is technically more available in pure form. The exothermicity of the reaction of nitrogen dioxide and alcohols to form alkyl nitrites and nitric acid is also less than that of nitrogen monoxide, oxygen and alcohols to form alkyl nitrites and water. With the elimination of the necessity of supplying oxygen and metering it as a function of the amount of nitrogen monoxide, fewer problems also result in the removal of the heat of reaction.
A disadvantage in the process described in U.S. Pat. No. 2,739,166, however, is that in the course of progressive reactions in the liquid phase, mixtures of alcohols and nitric acid result, the concentration of the latter increasing constantly. This is not only serious as regards safety aspects, the increased formation of secondary products, in particular the occurrence of toxic and highly explosive alkyl nitrates, must also be taken into account. It is also disadvantageous that the nitric acid residues occurring in the preparation of the alkyl nitrites contain at least still-unreacted alcohol, often also still-dissolved alkyl nitrite and also alkyl nitrate formed in secondary reactions and cannot be used further without complicated treatment. As regards the nitrogen dioxide used, this represents an economically unsatisfactory situation. There was therefore also the object, starting from technically readily available nitrogen dioxide, of developing a process for the preparation of alkyl nitrites which is suitable in particular for the synthesis of the technically interesting volatile compounds methyl nitrite and ethyl nitrite, does not lead to undesired secondary products, such as, for example, alkyl nitrates and provides the occurring nitric acid in a quality sufficient for further uses, in the most favourable case with the avoidance of additional cleaning measures. The far-reaching observance of the following requirements is essential for the successful operation of a process of this kind:
Nitrogen dioxide and the alcohol brought to reaction should as far as possible react completely with the formation of the desired alkyl nitrite and nitric acid. PA1 Reaction control and reaction technology should be designed such that the undesired formation of secondary products such as alkyl nitrates is suppressed and the desired alkyl nitrite is obtained in as far as possible pure form. PA1 The nitric acid resulting in the course of the reaction should, without an additional complicated operation such as, for example, an extraction, be separated from the desired alkyl nitrite completely and in a quality adequate for its further use. PA1 The heat of reaction liberated in the course of the reactions occurring within the reactor should be led off. PA1 As regards the safety requirements for the technical execution of the process to be sought, local overheatings and the occurrence of flammable mixtures must be avoided. PA1 As starting material it uses nitrogen dioxide such as occurs, for example, in industrial plant for the production of highly concentrated nitric acid and which is technically more available than nitrogen monoxide. PA1 It permits the use of water-containing alcohols without this having a negative effect on the quality of the alkyl nitrites prepared; with the aid of the water content of the added alcohol, the degree of dilution of the nitric acid obtained at the bottom of the column can be selectively adjusted within wide limits. PA1 It provides the nitric acid also occurring in the course of the reaction in high quality so that it can be used for further reactions without complicated purification. PA1 It is suitable in particular for the preparation of the technically interesting low-boiling alkyl nitrites, such as in particular methyl nitrite and ethyl nitrite, preferably methyl nitrite. PA1 In particular when water and/or inert gas are also used, it permits, by the consistent avoidance of flammable and explosive substance mixtures, substance concentrations and critical temperatures, all safety requirements to be complied with.